Many microorganisms which accumulate poly((R)-3-hydroxybutyric acid) in the microbial cells are known (see P. A. Holmes, Phys. Technol., Vol. 16, pp. 32-36 (1985) and Yoshiharu Doi, SEIBUNKAISEI KOBUNSHI ZAIRYO, pp. 26-30, Kogyo Chosakai (1990)). This polymer is a thermoplastic resin having biodegradability, i.e., enzymatic decomposability, hydrolyzability, and biocompatibility, and is now attracting attention as a functional polymeric material. For example, it is widely applicable as a clean plastic causing no environmental pollution, taking advantage of its degradability with microorganisms in soil or water (see Yoshiharu Doi, SEIBUNKAISEI KOBUNSHI ZAIRYO, pp. 19-26, Kogyo Chosakai (1990)).
Various biological or chemical processes for synthesizing poly(3-hydroxybutyric acid) on an industrial scale have hitherto been proposed. For example, known biological processes include a process of utilizing Azotobacter vinelandii as disclosed in JP-A-60-251889 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, biological synthesis of a polymer requires complicated steps for separation of the accumulated polymer from microbial cells, resulting in increased production cost. Besides, there is obtained a polymer having a specific steric configuration at an optical purity of 100%, i.e., an (R)-polymer. Such a polymer has a melting point as high as 180.degree. C. and is therefore difficult to handle as reported in Yoshiharu Doi, ibid, p. 26.
On the other hand, it is known that poly(3-hydroxybutyric acid) is produced by ring-opening polymerization of .beta.-butyrolactone. Examples of this chemical process include a process of using a triethylaluminum/water catalyst system (see Richard A. Gross, et al., Macromolecules, Vol. 21, pp. 2657-2668 (1988)), a process of using a diethylzinc/water catalyst system (see Y. Zhang, et al., Macromolecules, Vol. 23, pp. 3206-3212 (1990) and N. Tanahashi, et al., Macromolecules, Vol. 24, pp. 5732-5733 (1991)), a process of using an aluminum-porphyrin complex as a catalyst (see S. Asano, et al., Macromolecules, Vol. 18, pp. 2057-2061 (1985)), a process of using a potassium solution or other potassium compounds as a polymerization initiator (see Z. Jedlinski, et al., Macromolecules, Vol. 18, pp. 2679-2683 (1985)), a process of using a specific compound comprising aluminum and zinc (ZnAl.sub.2 O.sub.2 (OCHMe.sub.2).sub.4) as a polymerization initiator (see N. C. Billingham, et al., J. Organomet. Chem., Vol. 341, pp. 83-93 (1988)), and a process of using a metal alkoxide, e.g., a magnesium or tin alkoxide, as a polymerization initiator (see Hans R. Kricholdorf, et al., Macromolecules, Vol. 21, pp. 286-293 (1988)). In the above-mentioned process using a diethylzinc/water catalyst system, .beta.-butyrolactone to be ring-opening polymerized is used after dehydration with calcium hydride.
According to these chemical processes, complicated steps as involved in biological processes are not necessary, and the steric configuration of the produced polymer can be controlled to some extent by a choice of the steric configuration of the starting .beta.-butyrolactone. It is reported that an (R)-polymer produced undergoes biodegradation, while an (S)-polymer produced does not, so that the rate of decomposition can be controlled by the production ratio of the (R)-polymer (see Jhon E. Kemnitzer, et al., Macromolecules, Vol. 25, pp. 5927-5934 (1992)). In other words, chemical processes are expected to provide a polymer having an arbitrary optical purity according to use.
However, the conventional ring-opening polymerization of .beta.-butyrolactone encounters difficulty in providing poly(3-hydroxybutyric acid) having a high molecular weight, i.e., a number average molecular weight (Mn) of not less than 100,000 and a degree of polymerization of not less than about 1,000, as achieved by the biological processes. In addition, the catalysts or catalyst systems proposed to date exhibit unsatisfactory catalytic activity, still incurring high production cost.
In some detail, the above-mentioned process using a diethylzinc/water catalyst system produces a polymer having an Mn as low as 50,000 in a yield of about 84% at the highest through a reaction at 60.degree. C. for 5 days (see N. Tanahashi, et al., Macromolecules, Vol. 24, pp. 5732-5733 (1991)).
A 1,3-disubstituted-1,1,3,3-tetraalkyldistannoxane (hereinafter referred to as a distannoxane derivative), one of the tin compounds according to the present invention, has a unique ladder-like double structure and is known to be useful as a catalyst mainly for urethane synthesis or ester synthesis (see Junzo Otera, et al., NIPPON KAGAKUKAISHI, No. 6, pp. 601-610 (1990) and J. Otera, et al., J. Org. Chem., Vol. 56, pp. 5307-5311 (1991)).
A process for producing a lactone polymer comprising reacting a lactone with a compound having a hydroxyl group or an amino group in the presence of a distannoxane derivative is disclosed in JP-B-43-2947 (the term "JP-B" as used herein means an "examined Japanese patent publication"). The lactone used in this process has a 6- or higher-membered ring, and no mention is made of ring-opening polymerization of a 4-membered lactone which is deemed to be more difficult to be ring-opening polymerized than 6- or higher-membered lactones. Besides, all the polymers obtained have as low a molecular weight as 500 to 12000 (corresponding to a polymerization degree of about 5 to 140). Further, the process essentially needs a polymerization initiator, failing to meet the demands for simplification of production steps and reduction of production cost.
JP-B-46-645 (corresponding to U.S. Pat. No. 4,071,507) teaches a process for polymerizing a lactone monomer in the presence of a specific polymerization initiator and a catalyst, such as tetrabutyldichlorodistannoxate. However, no specific mention being made of .beta.-butyrolactone, the starting material used in the present invention, it is not construed that the outstanding problems have been settled sufficiently. Further, this patent publication says that a polymer having an average molecular weight of not less than 10,000 and, in some cases, 100,000 or even more can be obtained. However, the working examples of this patent publication simply describe the viscosity of the resulting lactone polymers without specifying the molecular weight, also failing to solve the problem. Furthermore, a specific polymerization initiator, such as glycol diacetate, is still required. Use of such a polymerization initiator not only conflicts with the demands for simplification of steps and reduction of cost but causes a reduction in degree of polymerization because the initiator acts as a catalyst poison in the polymerization system.